The present invention relates to a pre-amplifier for use in an optical receiver.
In recent years, a PON (passive optical network) system such as a PDS (passive double star) optical subscriber system in Japan has been variously examined for the purpose of realization of a future FTTH (fiber to the home) system. It is, however, economically difficult to introduce optical fibers into general public home as compared with the existing metallic subscriber network. Under such circumstances, an optical subscriber network, in which each of plural optical fibers connected with a base station is branched into plural optical fibers through a star coupler, is regarded to be a promising system. In this network, one subscriber is connected with each of the branched optical fibers. The subscribers thus share equipment of the base station, so that a bidirectional digital communication service can be inexpensively provided to the respective subscribers.
An optical receiver installed at home of each subscriber includes a photodiode for generating a current signal in accordance with an optical signal supplied through the optical fiber, a pre-amplifier for converting the current signal into a voltage signal, and an AGC (automatic gain control) circuit for converting the voltage signal into a voltage signal with a predetermined amplitude. A clock signal is recovered from the output of this AGC circuit, so that digital data can be reproduced in synchronization with the clock signal.
As a conventional pre-amplifier for use in an optical receiver, an amplifier including a feedback resistor interposed between input and output terminals of an inverting amplification circuit is known. When a gain of the inverting amplification circuit and a resistance value of the feedback resistor are indicated as A(s) and Rf, respectively, and an input current and an output voltage of the pre-amplifier are indicated as Iin(s) and Vout(s), respectively, a transfer function T(s) of the pre-amplifier is represented as follows: EQU T(s)=Vout(s)/Iin(s)=-A(s)/{sCin+{1+A(s)}/Rf} (1)
wherein s indicates a complex angular frequency, and Cin indicates an input capacitance, which corresponds to a sum of all stray capacitances parasitic on the input terminal of the inverting amplification circuit. When an open loop gain and a corner angular frequency of the inverting amplification circuit are indicated as Ao and .omega.h, the gain A(s) is linearly approximated as follows: EQU A(s)=Ao/(1+s/.omega.h) (2)
Accordingly, the following relationships hold: EQU T(s)=To/{1+2.zeta.s/.omega.n+s.sup.2 /.omega.n.sup.2 } (3) EQU .omega.c=(1+Ao)/(Cin Rf) (4) EQU To=-Ao Rf/(1+Ao) (5) EQU .omega.n=(.omega.h.omega.c).sup.1/2 (6) EQU .zeta.=(1+Cin Rf.omega.h).omega.n/{2(1+Ao).omega.h} EQU =(1+Cin Rf .omega.h){4Cin Rf (1+Ao).omega.h}.sup.-1/2 (7)
wherein .zeta. is a constant designated as an attenuation coefficient.
The formula (3) reveals that the transfer function T(s) is a reciprocal of a quadratic function of the complex angular frequency. Therefore, when the attenuation coefficient .zeta. is small, peaking can appear in the frequency characteristic of the pre-amplifier, which can sometimes cause oscillation.
A distance between the base station and the home of a subscriber, namely, the length of the optical fiber, varies depending upon the subscriber. Accordingly, in some cases, a subscriber P is provided with an optical signal with large strength but another subscriber Q is provided with merely a weak signal due to the attenuation of light in the optical fiber. As a countermeasure, the optical receiver of the subscriber P can be provided with an optical attenuator. However, when optical receivers having the same circuit configuration are to be installed in the home of all the subscribers for low cost system, the pre-amplifier is required to have a wide dynamic range characteristic so as to be able to deal with a wide range of currents from a minor current to a large current.
On the other hand, in order to improve the sensitivity of the optical receiver, it is necessary to decrease the influence of a thermal noise derived from the feedback resistor of the pre-amplifier. The equivalent input thermal noise &lt;IRf.sup.2 &gt; derived from the feedback resistor is represented as follows: EQU &lt;IRf.sup.2 &gt;=4kTB/Rf (8)
wherein k indicates the Boltzmann's constant, T indicates an absolute temperature and B indicates a frequency band. As is understood from the formula (8), the resistance value Rf of the feedback resistor is necessary to be increased in order to improve the receiving sensitivity by decreasing the influence of the thermal noise. However, in the case where the feedback resistance value Rf is increased, when the input current is large, the amplitude of the output voltage becomes so large and saturated that the waveform of the output voltage can be largely distorted. In other words, it is difficult to attain the wide dynamic range characteristic. In this manner, there is a trade-off relationship between the high sensitivity and the wide dynamic range characteristic, and hence, it is difficult to simultaneously attain the both advantages.
As a countermeasure for realizing both the high sensitivity and the wide dynamic range characteristic, Japanese Laid-Open Patent Publication No. 3-60208 discloses a pre-amplifier in which a diode is connected in parallel with a feedback resistor. In this pre-amplifier, when a voltage applied between the ends of the feedback resistor is increased because the input current is increased, the diode connected in parallel with the feedback resistor is turned on, so that a part of the current can be bypassed into the diode and that an effective feedback resistance value Rf can be decreased. As a result, excessive increase of the output voltage amplitude can be avoided.
In this pre-amplifier, however, turbulence of the output voltage waveform such as ringing can be disadvantageously easily caused. Specifically, when the diode is turned on in response to the increase of the input current, the effective feedback resistance value Rf is decreased so as to decrease the attenuation coefficient .zeta. (See the formula (7)). Accordingly, the peaking appears in the frequency characteristic of the pre-amplifier, resulting in making the operation of the pre-amplifier unstable.